Heat-resistant structures for protecting boiler tubes in waste heat boilers are well known in the prior art. For example, in Patent Publication 2-203194, as is shown in FIG. 9, boiler tubes 101 are connected by fins 102 to form tube walls 100. Panels 104 are constructed of heat-resistant brick. The areas between the upper surfaces of panels 104 and the surfaces of tube walls 100 which face towards the gas and the areas between each pair of adjacent tubes are filled with mortar 107. The panels are fixed to tube walls 100 by means of stud bolt 103. The end of the aforesaid stud bolt 103 which is exposed is covered by cap 110, also constructed of heat-resistant brick.
This prior art design requires that a depression 109 be provided in heat-resistant panel 104 to contain cap 110. This resulted in an extremely thick panel 104 with very low heat conductivity.
This problem was addressed in Japanese Patent Publication 4-227401, wherein the design pictured in FIG. 8 was proposed. A heat-resistant block provides a composite structure for the boiler tubes while insuring a good conductive transfer of heat. To explain FIG. 8 more fully, it shows a heat-resistant design for a set of boiler tubes to recover the waste heat from a garbage incinerator. This design protects the tubes from both the heat of the combustion exhaust gases and the corrosive atmosphere.
In the drawing, 11 are boiler tubes and 13 are flat ribs to lend strength to the tubes 11 by connecting them in either a horizontal or a vertical array. 12 is a boiler tube assembly consisting of a number of rows of tubes 11 and the flat ribs 13 which connect the adjacent tubes 11 in either a horizontal or a vertical array.
Reference numeral 26 identifies heat-resistant blocks of a ceramic material which are placed so as to protect the aforesaid tubes 11 from the combustion gases. The aforesaid tubes 11 are protected from the heat of the combustion exhaust gases and the corrosive atmosphere they create by the heat-resistant blocks 26.
23a is a bolt to affix the aforesaid block 26 onto one of the flat ribs 13. The bolt extends from rib 13 through heat-resistant block 26. When nut 23b is tightened, block 26 is fastened to tubes 11 and rib 13. 20 is mortar which fills the spaces between the heat-resistant block 26 and tubes 11 or ribs 13.
In the heat-resistant assembly of the prior art boiler described above, block 26 is composed of a highly heat-resistant ceramic, and the space between tubes 11 or ribs 13 and block 26 is filled with mortar 20. This promotes the flow of heat while protecting tubes 11 from the combustion gases and their corrosive atmosphere.
However, because the aforesaid prior art block is composed of a ceramic, its heat conductivity is relatively high. If only the portion around nut 23b is indented, and surface 26a, which faces the combustion gases, is to be flat, block 26 will necessarily have to be quite thick. As a result, boilers with the prior art design were not able to achieve the maximum heat flow which is essential to boiler efficiency.
Furthermore, because heat is not transferred efficiently from block 26 to tubes 11, a large temperature differential occurs between the interior and exterior of block 26, and the temperature of surface 26a, the surface which is exposed to the combustion gases, gets quite high. This results in a thermal expansion differential between tubes 11 (which are composed of a heat-resistant metal) and heat-resistant block 26 (which is composed of a ceramic). The end result is that block 26 experiences high thermal stress.
If the ideal conditions are not met for heat transfer between the assembly of block 26 and tubes 11, the temperature of the exterior surface of block 26 will increase, and the residual ash 24 of the combusted fuel will melt and adhere to the block, forming a layer of thermal insulation.
Because the heat transfer capability of this layer of ash is extremely inadequate, once ash 24 begins to adhere, the further melting and buildup of ash is promoted and the layer of insulation becomes thicker and thicker, posing a significant obstacle to heat transfer. And because ash 24 contains corrosive components such as chlorine compounds, tubes 11 are exposed to high-temperature corrosion which may result in damage.
The aforesaid heat-resistant block 26 is attached to tubes 11 and flat ribs 13 by bolt 23, which is fixed to one of the flat ribs 13. The compression which occurs when the bolt 23 is tightened and the difference in thermal expansion between tubes 11 (which are composed of heat-resistant metal) and block 26 (which is composed of a ceramic) may result in thermal warping. This stress and the thermal stress due to the temperature differential between the interior and exterior of block 26 may result in damage to the block.
The aforesaid bolt 23a and nut 23b are exposed to the combustion gases, which are liable to corrode them. If the corrosion is allowed to proceed, heat-resistant block 26 may be damaged or fall away from the boiler tubes.